1. Field of the Invention
This invention relates to antennas and more particularly to foreshortened monopole and dipole antennas.
2. Description of the Prior Art
The physical length of a linear monopole or dipole antenna is inversely proportional to its operating frequency. Monopoles or dipoles designed for use at frequencies at or above the UHF range (i.e. &gt;300 MHz) have overall lengths that are relatively short. For instance a monopole antenna operating at 300 MHz is approximately only 10 inches in length. However, there are many applications where the length of UHF or microwave frequency dipole or monopole antennas are too large for certain applications and may need to be reduced by 50% or more. For operation in the HF or low VHF frequency range, the length of the linear monpole or dipole can be reduced up to 50% by inductive loading. The inductive loading of the monopole or dipole antenna is accomplished by physically inserting inductors at various parts on the linear monpole or dipole. For frequencies above the UHF range, this reduction technique becomes impractical because the size of the inductors or capacitors is not negligible compared to the relatively small size of the dipole or monopole. In addition, it is difficult to obtain an inductor which will provide the exact amount of inductance without stray capacitance at microwave frequencies. For this reason, the foreshortening of the microwave monopole or dipole antennas is accomplished without using actual inductor or capacitors.
A technique which can be used to reduce the length of a monopole or dipole up to 45% is described in U.S. Pat. No. 3,732,572. This patent describes a log periodic dipole antenna (LPDA) in which certain radiating elements (dipoles) are foreshortened in order to conserve space without adversely affecting antenna performance. This is accomplished by configuring each size-reduced dipole with the interior profile of a double ridge waveguide. However, when these foreshortened dipoles are used as radiation elements in a practical commonly used log periodic dipole antenna, the width of such antennas cannot be reduced as much. The reason for this is that these antennas have taper angles of approximately 25%. Therefore the available spacing between adjacent dipole elements is relatively small and this prevents the use of foreshortened dipole elements with large width-to-length (B/A) ratios when the planes of these elements are parallel to the antenna axis. For example, the width of a log periodic dipole antenna with a taper angle of 20% can be reduced up to 30%, and one with a 30% taper angle can only be reduced by approximately 25%. Log periodic dipole antennas having a very small taper angle (less than 10%), and therefore characterized by long structures and large spacings between adjacent dipole elements, can be reduced up to 40% because foreshortened dipoles with large B/A ratios can be used. Nevertheless the 40% reduction in antenna width is still not sufficient to enable the antenna to fit into the space available in many applications.
A well known method of reducing the length of the linear monopole or dipole antenna is the top disk loading technique. A monopole version of this antenna consists of two parts:
1. a wire stem with a length of (a), and
2. a loading disk with a diameter of (d) mounted on the stem.
The reduction factor of the disk antenna is directly proportional to the size (surface area) of the disk if the length of the stem is kept constant. The length of a dipole or monopole can be reduced by 70% by using top disk loading. However, for reduction greater than 50%, (d) becomes greater than (a). This configuration, a three dimensional body, is structurally very difficult to use with either the LPDA's or the Yagi antennas. Furthermore, when the disk antenna is used as a single, stand alone antenna, it is difficult for an antenna designer to know whether to treat it as a monopole or a disk antenna, and it is very difficult to support it because it because of the top loading.
This invention is directed to an improved foreshortened monopole or dipole antenna construction which overcomes these limitations.